The invention relates generally to antifreeze compositions and more particularly to corrosion inhibitor packages for antifreeze compositions. Antifreeze compositions containing these packages are particularly suitable for use in closed systems, such as closed loop heat exchange system, more particularly, hydronic heating and cooling closed loop systems containing aluminum.
Hot water boiler systems often use a heat transfer medium such as a fluid comprising water or a water-glycol mixture, such as a water-alkyleneglycol mixture, and an antifreeze or anti-corrosion package. As used herein, “package” will refer to a combination of additives. The medium is used to transfer heat between the source of the heat, e.g. a cast metal heat exchanger and the system, e.g. base board heaters or radiators, designed to deliver heat throughout an area, such as a house. Although the heat transfer fluid provides a means to transfer heat, the fluid can cause corrosion on the surface of the metals in the boiler, transfer conduits and the metal heat exchanger.
Historically, cast iron heat exchangers have been the choice of the heating industry. However, because cast iron heat exchangers are substantially heavier and are less efficient than aluminum at exchanging heat, boilers and other heating and cooling systems containing aluminum fluid conduits are gaining popularity and usage. Aluminum heat exchangers and the like, however, can exhibit undesirable corrosion problems under certain adverse conditions.
Water or water-glycol mixture heat transfer fluids commonly cause corrosion, especially in metal systems, which are particularly susceptible to corrosion. Corrosion can be accelerated by high temperatures and pressures, which are common in an operating boiler system, as well as by minerals or other corrosive species found in water used in boilers.
The industry has long used anti-corrosion packages to provide added protection to the metal surfaces. A preferred corrosion inhibitor for propylene glycol based fluids has been dipotassium phosphate at levels ranging from 0.5% to 5% by weight, often referred to as Inorganic Additive Technology (IAT), such as that available from Hercules Chemical Company, Passic, N.J., Third Coast Chemicals, Pearland, Tex. or Dow Chemical Co., Dow Frost HD MSDS. With this combination of corrosion inhibitor and an alkylene glycol/water mixture, sufficient corrosion protection can commonly be provided to pass the ASTM D-1384 test method. The alkylene glycol/water mixture can include, but is not limited to, ethylene, propylene or dipropylene glycols. Alternate IAT types other than dipotassium phosphate include but are not limited to borates, nitrates, molybdates, nitrites, and silicates, which are consumed as they perform their function, such as balancing the pH. Inorganic additives can also combine with impurities in the formulation or on the surface of the metal and thus be transformed or consumed. These alternate IAT types identified are not used often in heat transfer fluids for boilers because of their toxicity or other chemical stability shortcomings.
The standard ASTM D-1384 corrosion test is a screening test, which measures the corrosion protection provided by alkylene glycol solutions, such as propylene glycol, on standard metals under specific conditions. The corrosion test results are expressed in weight loss in milligrams, representative of mils of penetration per year. The conditions under which the tests are conducted include glycol solutions held at 190° F. (88° C.) for two weeks in suitable glassware, where the glycol level is set at 30% glycol by volume. Metal coupons tested include copper, solder, brass, mild steel, cast iron and aluminum. The limits for weight loss under the test methods described for D-1384 are cited in ASTM D-3306, where the ASTM Limit for each metal is copper (10 mgms), solder (30 mgms), brass (10 mgms), steel (10 mgms), cast iron (10 mgms) and aluminum (30 mgms). The copper coupon conforms to UNS C11000 (SAE CA110), solder conforms to Alloy Grade 30A (SAE 3A), steel conforms to UNS G10200 (SAE 1020) with the chemical composition of the carbon steel is as follows: carbon, 0.17 to 0.23%; manganese, 0.30 to 0.60%; phosphorus, 0.040% maximum; sulfur, 0.05% maximum; cast iron conforms to Alloy UNS F 10007 (SAE G3500) and cast aluminum conforms to Alloy UNS A23190 (SAE 329). In this test method, specimens of metals are totally immersed in aerated coolant solutions prepared with corrosive salts for 336 hours at 190° F. (88° C.). Each test is run in triplicate and the average weight change determined for each metal.
Cast metal heat exchangers used in boilers have traditionally been made of cast iron. Corrosion inhibitor packages based on dipotassium phosphate have long been used in cast iron heat exchangers. However, with the emergence of cast aluminum heat exchangers there is a need to develop a new corrosion inhibitor package for a variety of reasons. First, suppliers of propylene glycol antifreeze post an aluminum disclaimer on their products. Although based on the ASTM D-1384 method, aluminum loss is within the limits set by D-3306, antifreeze suppliers continue to utilize a disclaimer with respect to aluminum surfaces. This has caused concerns as to their effectiveness for use with aluminum. Second, when tested with other methods, such as ASTM D-6208, which measures repassivation, i. e. resistanse to chemical pitting of aluminum surfaces by galvanostatic measurement, a test favored by manufacturers of aluminum combustion engines, the propylene glycol-DKP solutions do not meet the passing value of- 400 mV. Third, when tested against D-4340, which measures weight loss under specific use conditions of heat and motion of the fluid, propylene glycol-DKP solutions also fail the standard. Fourth, the extensive use of complex inhibitor packages used in aluminum automobile engines is not suitable for use in hot water boilers.
ASTM D-6208 is a test method designed to measure the relative effectiveness of corrosion inhibitors to mitigate pitting corrosion of aluminum and its alloys. The minimum potential number derived is a measure of the protection against continued pitting corrosion. The test is aggressive in that the standard solution contains chloride, sulfate and bicarbonate. ASTM D-4340 evaluates the effectiveness of heat transfer fluids in combating corrosion under conditions that may exist in aluminum engines.
Various attempts to address these and other drawbacks of conventional antifreeze solutions are disclosed in U.S. Pat. Nos. 6,398,984, 6,391,257, 6,290,870, 6,143,243, 5,766,506, 5,330,670, 5,290,468, 5,269,956, 5,242,621, 5,085,793, 5,085,791, 4,946,616, 4,873,011, 4,851,145, 4,873,011, 4, 647,392, 4,588,513, 4,452,758, 4,426,309, 4,382,870 and 4,320,023, which are all incorporated in their entirety herein by reference.
Aluminum surfaces are susceptible to several types of corrosion mechanisms, including general corrosion, pitting, crevice and cavitation corrosion, as discussed in U.S. Pat. No. 6,398,984, which has been incorporated herein by reference. Complex mixtures of triazoles, thiazoles, borates, silicates, phosphates, benzoates, nitrates, nitrites and molybdates have been used as corrosion inhibitors in antifreeze solutions for automobile engines. More detail regarding mixtures for internal combustion engines is available in U.S. Pat. Nos. 4,873,011 and 4,946,616, which are incorporated in their entirety herein by reference. These complex mixtures, however, are not suitable for protecting the heat exchangers used in boilers from corrosion. For example, they can materially alter the surface characteristics of the metal involved in the heat exchanger. This can significantly reduce the efficiency of heat transfer. Some of the components also have a tendency to form gels or thick layers on the metal surfaces. Also, these mixtures can be expensive to use in formulations for the average home boiler and do not provide protection over a sufficiently long period of years. Additionally, because of the risk of contamination of the water from the heater side to the circulating hot water side of the boiler, the toxicity of the heat transfer fluid plays a role in determining which additives may be used.
The emergence of the aluminum automobile engine has prompted the development of new corrosion additives for cast aluminum engines, as discussed in U.S. Pat. No. 6,391,257, which is incorporated it its entirety herein by reference. The automotive industry has developed engine coolants based on mono and dicarboxylic acid technology used in conjunction with other traditional additives, as illustrated in U.S. Pat. No. 4,647,392, which is incorporated herein by reference in its entirety. This is referred to as Organic Acid Technology (OAT), which includes carboxylate salt technologies, which are not consumed as a part of their use, thereby extending the life of the antifreeze. Carboxylates protect metal surfaces by application of a thin coating, and are often used in combination with IAT's, but have restricted use because some OAT's react with IAT's. Although carboxylic acids have been successfully used either among themselves or in combination with other additives, when formulated with propylene glycol, they are not known to pass either the ASTM D-1384 or D-6208.
In light of shortcomings of the conventional methods and applications known in the art, it is desirable to provide improved additives for antifreeze compositions.